EKW-73
Rinsing with Less Water
2-77?y
by William Yates
Poly Products Corp., Atwood, CA
INTRODUCTION
ow important is rinsing? In the
typical nickel-chrome line, it is
not unusual to find three out of four
tanks devoted to rinsing. This vital step
after each process is necessary to:
1. Stop the action of a chemical solution.
2. Prevent the contamination of a
subsequent process.
3. Prevent chemical staining of the
work.
Good rinsing with small amounts of
water has always been a sought after
goal, but almost imperative in today’s
regulationdominatedenvironment. Reducing the flow of water lessens the
load on waste treatment systems and
makes dragout recovery far more
economical and practical.
Rinsing is generally accomplished
by immersing processed work in a tank
of relatively clean running water. This
results in the dilution bf the residue on
the work’s surface. In a continuous
processing line, the dilution ratio for a
single rinse is easily calculated using
the following formula:
Dilution ratio =
dragin (gph)
water flow (gph)
dragin (gph)
(We are using “gallons per hour” as an
easily handled unit of measurement.)
The rinse ratio number can be used
to determine the concentration of any
of the dragin components in this rinse
tank.
H
+
EXAMPLE:
Nickel plated parts are rinsed in a
single tank of running water. Dragout
from the nickel tank is estimated at 1
gph, water flow through the singlerinse
is 2,000 gph. The nickel metal concentration in the process tank is 10 oz/gal.
Dilution ratio =
1 gph dragin 2,000 gph water -
+
1 gph dragin
2,001
~-
- 2,001:l
1
Nickel metal concentration in the rinse
tank is
50
10oz per hour dragin 2,001
0.005 odgal
Here we should note:
1. The generally accepted level of
contamination in the final rinse is 0.005
odgal metal or cyanide or 0.002 ozlgal
chrome metal.
2. The 2,000 gph to the rinse tank
in the example is an unacceptably high
flow rate.
THE MAGIC OF COUNTERFLOW
RINSING
The only purpose for counterflowing
rinses is to conserve water. By increasing from one to two rinses, a water
savings of 20 to 100 times is realized.
In the nickel rinsing example above,
2,000 gallons per hour of rinse water
is needed to maintain the low nickel
concentration of 0.005 ozlgal in the
rinse tank.The same rinsing (dilution
ratio) can be accomplished with:
2 counterflowing rinses and 44 gph
flow rate
3 counterflowing rinses and 12 gph
flow rate
4 counterflowing rinses and 6.4 gph
flow rate.
Although a small amount of dilution
occurs in each rinse tank, the multiplication of these ratios produces a high
total. The formula for the riilse ratio in
a multiple counterflow arrangement is:
Rinse ratio
=
Where “RW’ is rinse water, “DO’ is
dragout (both are in gph), and “n” is
the number of rinses.
An easier way to closely approximate the rinse ratio is:
rinse water flow
Dragout (gp;yh)
(
MORE PRACTICE:
The dragout from a 32 oz/gal decorative chrome bath is estimated at 1 gph.
Chromic acid is about 52% metal; the
actual metal content is about 16 oz/gal
in this bath. In a triple counterflowrinse
system, the third rinse should have no
more than 0.002 ozlgal metal. This requires a rinse ratio of 8,000:l.
16oz/gal
= 8,000
0.002 oz/gal
Fooling around with a pocket calculator, the cube root of 8,000 is 20.
20 x 20 x 20 = 8,000
With 20 gph water flowing through
the three counterflowing rinses, the
rinse ratio is about 8,000:l. Each rinse
dilutes the dragout to about 1/20 of the
concentration of the previous tank
(Rinse 1 is 1/20 of the process bath;
Rinse 2 is 1/20 of Rinse 1; Rinse 3 is
1/20 of Rinse 2).
Most of us are used to seeing much
larger rinse ratios with triple counterflow rinses operating at flow rates
of 5 gallons per minute, or 300 gph.
If the dragout volume is 1 gph, then
there is an approximate dilution of
300:l in each of the three rinses, or
27,000,000:l total! This would put the
metal concentration in the chrome
example at 0.0000006ozlgal in the last
rinse, or more than 3,000 times cleaner
than necessary. Nice, but very costly
in water usage.
Table 1. Rinse Water Requirements
Dragout
Rinse Water Requirements
Concentration
(ozlgal)
(gph)
for 1 gph Dragout.
CountetflowingRinses
Chrome Other
Metal Metals
1
2
4
8
16
32
2.5
5.0
10.0
20.0
40.0
80.0
One
Rinse
2
500
1,000
2,000
4,000
8,000
16,000
22
32
44
63
89
126
3
4
8 4.4
10 5.4
12 6.4
16 7.7
20 9.2
25 11
5
3.2
3.7
4.3
5.0
5.8
6.7
‘For 2 gph dragout, muitiply by two, for 1h gph multiply by 0.5, etc.
Table I shows the water requirements
for one gallon dragout of various concentrations and for various numbers of
rinses. These flow rates will result in
a final rinse concentration of 0.002 OZ/
gal chrome metal or 0.005 oz/gal other
metals (nickel, zinc, etc.). This table
illustrates several important points:
0 Rinse water requirements increase in
direct proportion to dragout volume,
METAL FINISHING
regardless of the number of rinse stations.
0 Rinse water requirements for a single
rinse station increase in direct proportion to dragout concentration.
0 Rinse water flow rate requirements
drop off dramatically with the addition of counterflow rinse stations.
0 In this table, the greatest change in
flow rate is between one and two
rinses; the smallest change is between 4 and 5 rinses.
0 When using multiple counterflowing
rinses, it is more beneficial to reduce
dragout volume than dragout concentration.
All these figures and calculationsdepend on the complete mixing of the
dragout (or dragin) with the water in
each rinse station.
AIR AGITATION
One way to achieve good mixing of
rinse water and dragin is to use air agitation to keep the water in motion
within the rinse tank. Place the air agitation pipe across the bottom of the tank
and close to the wall or partition on the
water inlet side. This will provide a
rolling motion to the rinse water, pushing any floating films across to the discharge side of the tank.
When processing barrels or parts that
entrap solution, special care in rinsing
is required. It is important to flush out
the interiors of barrels and entrapping
parts with several immersions in each
rinse station.
SPRAYS
Spray rinses can be added to the
processing line to increase the number
of counterflowing stations. Sprays may
be positioned to rinse racked parts as
they leave any rinse station or as they
emerge from the plating or processing
tank. Each set of sprays should be
supplied with water pumped from the
following rinse tank. There are two advantages of spraying over the final rinse
tank: city water line pressure eliminates
the need for a pump, and any overspray
beyond the rinse tank is free of dragout.
Pumps or city water pressure should be
activated only when processed parts are
actually in the spray zone. This will
conserve water and allow the use of
higher flow rate nozzles.
factor of 10. This was done by using
several counterflowing rinses after each
of the processes: cleaner, acid, nickel
and chrome. This made evaporative recovery possible of all nickel and
chrome dragout, with savings of chemical and waste treatment costs. More
water was saved by re-using the acid
rinse water in rinses after the cleaner.
THE REAL WORLD
One large nickel-chrome plating
shop cut their water consumption by a
CONCLUSION
Taking a little time to study your
MAY 1989
rinsing needs, and devising ways to
minimize water usage will pay great
dividends.
MF
Biography
Bill Yates is President of Poly Products Corp., a manufacturer of atmospheric evaporators and rinse water controlling devices. For the last 27 years,
he has been involved in engineering
and marketing of plating equipment.
He attended Wayne State University
and the University of Detroit.
51